Serves 4.  Great for a BBQ.  This is from a great food blog.

4 large Portobello mushrooms (get the biggest ones you can find)
12 oz. mild or hot Italian sausage
1-2 T olive oil (for browning sausage and onions and brushing on mushrooms before grilling)
1 small onion, finely chopped
1 tsp. minced fresh garlic
1/2 cup tomato sauce or marinara sauce
pinch red pepper flakes (optional)
8 oz. coarsely chopped spinach leaves
2 T chopped fresh basil
2 T chopped fresh parsley
3/4 cup freshly grated parmesan cheese

3/4 cup grated mozzarella, plus 1/2 cup for sprinkling on topPreheat gas or charcoal grill to high heat. Heat a small amount of olive oil in heavy frying pan, then crumble Italian sausage into pan and cook about 5 minutes, breaking apart with back of the turner as it cooks. When sausage is well-browned, remove to bowl.

If needed, add a bit more olive oil, then saute onions until they’re starting to soften, about 2 minutes. Add garlic and continue cooking until onions are starting to brown, a few minutes more. Add Italian sausage back into pan, then add red pepper flakes and tomato sauce and simmer 2-3 minutes while you chop the spinach.

Add chopped spinach and let it wilt and cook down, stirring so it’s evenly distributed in the sausage/onion mixture. Turn off heat and stir in 1/2 cup mozzarella, 1/2 cup grated parmesan cheese and chopped basil and parsley.

Remove stems from mushrooms caps, being careful not to break them then brush both sides of mushrooms with olive oil. Put mushrooms on grill, inside facing down, and cook about 4 minutes, until mushroom is slightly browned and starting to soften.

Remove mushrooms from the grill. (I put them on a wooden cutting board.) Carefully stuff the inside of each mushroom with sausage/spinach/cheese mixture, then top each mushroom with about 2 T grated mozzarella and parmesan cheese.

Carefully put mushrooms back on grill, stuffing facing up of course! Cover grill and cook about 5 minutes, checking often and removing from grill when mushrooms are cooked through, and cheese is melted and starting to brown, and stuffing mixture is hot. Serve immediately.